The metal-semiconductor junction plays an important role in all solid state devices. A metal-semiconductor junction can be either a rectifying contact or an ohmic contact. Ideally, an ohmic contact displays a linear relationship between current and voltage. A rectifying contact displays a superlinear relationship between current and voltage. In practice, a rectifying contact may be used to create a diode. A diode acts as a sort of electronic valve, allowing a large amount of current to flow in one direction of a circuit, while allowing a negligible amount of current to flow in the opposite direction. A diode requires a certain amount of voltage to be applied across it before significant current will flow. This voltage is referred to as the “forward operating voltage”, Vf and is defined as the forward bias required to pass a specified forward current (or current density) through the diode. While a diode generally will not allow significant current flow in the reverse direction, if a large enough reverse bias is applied, the diode will allow significant amounts of current to flow in the reverse direction. This reverse bias is commonly referred to as the breakdown voltage and is defined to be the voltage at which a specified reverse current (or current density) is passed through the diode.
One type of diode, commonly called the Schottky diode, is made from the junction of a metal and a semiconductor. A typical Schottky diode consists of a semiconductor sandwiched between two different metals. One metal forms an ohmic contact to the semiconductor, while the other metal forms a rectifying contact to the semiconductor.
Semiconductors utilize electrons and holes as carriers. A semiconductor that has electrons as the majority carrier is usually referred to as an n-type semiconductor, or as having electron-type conductivity. A semiconductor that has holes as the majority carrier is usually referred to as a p-type semiconductor, or as having hole-type conductivity.
For electron transport in an organic semiconductor, a rectifying contact is formed when the Fermi energy of the metal is lower than the energy of the conduction band edge of the semiconductor. The conduction band edge is also commonly referred to as the Lowest Unoccupied Molecular Orbital (LUMO) of the semiconductor. An ohmic contact for electron transport in an organic semiconductor is formed when the Fermi energy of the metal is higher than the conduction band edge of the semiconductor. Alternatively, an ohmic contact may be formed by heavily n-type doping the semiconductor adjacent the ohmic contact metal. (See S. M. Sze, Physics of Semiconductor Devices, 1981). The opposite situation applies to hole transport in organic semiconductors. An ohmic contact for hole transport in an organic semiconductor is formed when the Fermi energy of the metal is lower than the energy of the valence band edge, also referred to as the Highest Occupied Molecular Orbital (HOMO), of the semiconductor. An ohmic contact for hole transport may also be formed by heavily p-type doping the semiconductor adjacent the ohmic contact metal. A rectifying contact is formed for hole transport in an organic semiconductor when the Fermi energy of the metal is higher than energy of the valence band edge of the semiconductor.
Traditionally, inorganic silicon and gallium arsenide semiconductors have dominated the semiconductor industry. In recent years, however, there has been an increasing desire to use organic semiconductors as an alternative to the traditional inorganic semiconductors. One organic semiconductor is pentacene, a π-conjugated molecule. In its polycrystalline form, pentacene has relatively high hole mobility parallel to the surface of film for an organic semiconductor. The valence band edge of pentacene is about 4.9 eV below the vacuum level. Therefore, gold, with a work function of 5.1 eV, forms an ohmic contact for holes with pentacene, while aluminum, with a work function of 4.3 eV, creates a rectifying contact for holes with pentacene.
Schottky diodes have been made using organic semiconductors, including pentacene (Y. S Lee, J. H Park, J. S. Choi, Optical Materials, 21, 433–437, (2002)). However, unlike inorganic semiconductors, organic semiconductors are not usually doped in order to achieve their carrier transport properties. Controlled doping to influence the electrical properties of organic transport layers is a new development. Substances like polycrystalline phthalocyananines and amorphous 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine (TDATA) can be doped with the strong organic acceptor tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), resulting in conductivity much larger than the undoped material. (M. Pfeffer, A. Beyer, T. Fritz, K. Leo, Appl. Phys. Lett, 7, 729, (1998)).
Organic light-emitting diodes (OLEDs) have received much attention in recent years. Very low operating voltage multi-layered OLEDs have been achieved by combining a thick doped hole transport layer of TDATA with a thin undoped buffer layer of triphenyl-diamine (TPD). This structure resulted in OLEDs with a lower operating voltage and an improved electroluminescent efficiency compared to undoped devices. (X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, K. Leo, Appl. Phys. Lett, 78 410–412, (2001)).
Use of doped amorphous materials has also been applied to metal/intrinsic/p-doped (Mip) diodes using 4,4′,4″-tris(3-methylphenylphenyl amino) triphenyl-amine (MTDATA) in its intrinsic form, as well as doped with F4-TCNQ. Conductivity was shown to increase with increased doping of MTDATA, and both the breakdown voltage and forward voltage of the Mip diode could be increased by thickening the intrinsic layer. (J. Dreschel, M. Pfeiffer, X. Zhou, A. Nollau, K. Leo, Synthetic Metals 127, 201–05, (2002)).
A typical organic Schottky diode, such as one made with gold, pentacene and aluminum, exhibits three significant characteristics: (1) it has a low forward voltage in the forward-bias mode; (2) it has a low breakdown voltage in the reverse bias mode; and (3) the device is not robust, and it is easy to get short circuits in practical applications. For certain electronic applications, such as radio frequency identification (RFID) tags, a much higher breakdown voltage is required, while maintaining a relatively low forward voltage. Also, more robust devices that do not get short circuits are also desirable.